The primary goal of the Center is to pursue research and design activities in the well recognized and challenging areas of the renewable energy, more specifically wind energy, that are currently being supported by the U.S. Department of Energy (DOE), and the pertinent industries such as General Electric (GE). The subsequent section presents a list of these areas along with a summary description of the research/design work.
Future improvements to turbine components
CARE focuses and aligns its activities toward finding innovative solutions for the outstanding challenges that have been encountered and recognized by the researchers/designers who have been working in the renewable energy communities. Many necessary technological advances are already in the active development stages. Substantial research progress has been documented, and individual companies are beginning the development process for these technologies. The risk of introducing new technology at the same time that manufacturing production is scaling up and accelerating to unprecedented levels is not trivial. Innovation always carries risk. Before turbine manufacturers can stake the next product on a new feature, the performance of that innovation needs to be firmly established and the durability needs to be characterized as well as possible. These risks are mitigated by RD&D investment, including extensive component and prototype testing before deployment.
The following are brief summaries of key wind energy technologies that are expected to increase productivity through better efficiency, enhanced energy capture, and improved reliability.
The number one target for advancement is the means by which the energy is initially captured—the rotor. No indicators currently suggest that rotor design novelties are on their way, but there are considerable incentives to use better materials and innovative controls to build enlarged rotors that sweep a greater area for the same or lower loads. Two approaches are being developed and tested to either reduce load levels or create load-resistant designs. The first approach is to use the blades themselves to attenuate both gravity-and turbulence-driven loads. The second approach lies in an active control that senses rotor loads and actively suppresses the loads transferred from the rotor to the rest of the turbine structure. These improvements will allow the rotor to grow larger and capture more energy without changing the balance of the system. They will also improve energy capture for a given capacity, thereby increasing the capacity factor.
Another innovation already being evaluated at a smaller scale by Energy Unlimited Inc. (EUI; Boise, Idaho) is a variable-diameter rotor that could significantly increase capacity factor. Such a rotor has a large area to capture more energy in low winds and a system to reduce the size of the rotor to protect the system in high winds. Although this is still considered a very high-risk option because of the difficulty of building such a blade without excessive weight, it does provide a completely different path to a very high capacity factor (EUI 2003). CARE will continue working on finding innovative solutions for increasing the capacity factor of typical large turbines. Here, it should be noted that the wind tower that has been designed by Dr. Rashidi of CSU has achieved this increase in the capacity factor of turbines that produce electric power in the order of up to 0.1 Mega Watt.
Larger rotors with longer blades sweep a greater area, increasing energy capture. Simply lengthening a blade without changing the fundamental design, however, would make the blade much heavier. In addition, the blade would incur greater structural loads because of its weight and longer moment arm. Blade weight and resultant gravity-induced loads can be controlled by using advanced materials with higher strength-to-weight ratios. Because high-performance materials such as carbon fibers are more expensive, they would be included in the design only when the payoff is maximized. These innovative airfoil shapes hold the promise of maintaining excellent power performance, but have yet to be demonstrated in full-scale operation. The Center will employ the computational tools available at CSU and Ohio State University to perform state-of-the-art finite element analysis in design of innovative large scale blade systems, and will examine the ensuing vibratory and dynamic behavior of the large scale blades.
To date, there has been little innovation in the tower, which is one of the more mundane components of a wind installation. But because placing the rotor at a higher elevation is beneficial and because the cost of steel continues to rise rapidly, it is highly likely that this component will be examined more closely in the future, especially for regions of higher than average wind shear. The Center will also focus on designing of innovative tower systems for the large scale (Mega Watt level) turbines.
The Drive-train (Gearbox, Generator, and Power Conversion)
Parasitic losses in generator windings, power electronics, gears and bearings, and other electrical devices are individually quite small. When summed over the entire system, however, these losses add up to significant numbers. Improvements that remove or reduce the fixed losses during low power generation are likely to have an important impact on raising the capacity factor and reducing cost. These improvements could include innovative power-electronic architectures and large-scale use of permanent-magnet generators. Direct-drive systems also meet this goal by eliminating gear losses. Modular (transportable) versions of these large generation systems that are easier to maintain will go a long way toward increasing the productivity of the low-wind portion of the power curve.
Currently, gearbox reliability is a major issue, and gearbox replacement is quite expensive. One solution is a direct-drive power train that entirely eliminates the gearbox. This approach, which was successfully adopted in the 1990s by Enercon-GmbH (Aurich, Germany), is being examined by other turbine manufacturers. A less radical alternative reduces the number of stages in the gearbox from three to two or even one, which enhances reliability by reducing the parts count. The fundamental gearbox topology can also be improved, as Clipper Wind-power (Carpinteria, California) did with its highly innovative multiple-drive-path gearbox, which divides mechanical power among four generators. The multiple-drive-path design radically decreases individual gearbox component loads, which reduces gearbox weight and size, eases erection and maintenance demands, and improves reliability by employing inherent redundancies. CARE will work on the innovative design concepts for running plurality of electric generators that are all simultaneously rub by a large scale rotor system.
A study completed by BEW Engineering (San Ramon, California; Behnke, Erdman, and Whitaker Engineering 2006) shows that using medium-voltage power systems for multi-megawatt turbines could reduce the cost, weight, and volume of turbine electrical components as well as reduce electrical losses. The Center will work on innovative design ideas to take advantage of medium-voltage power systems for multi-megawatt turbines.